Neutron-scattering Experiments Research Articles

Alkali hydroxide (LiOH, NaOH, KOH) in water: Structural and vibrational properties, including neutron scattering results.

Structural and vibrational properties of aqueous solutions of alkali hydroxides (LiOH, NaOH, and KOH) are computed using quantum molecular dynamics simulations for solute concentrations ranging between 1 and 10M. Element-resolved partial radial distribution functions, neutron and x-ray structure factors, and angular distribution functions are computed for the three hydroxide solutions as a function of concentration. The vibrational spectra and frequency-dependent conductivity are computed from the Fourier transforms of velocity autocorrelation and current autocorrelation functions. Our results for the structure are validated with the available neutron data for 17M concentration of NaOH in water [Semrouni et al., Phys. Chem. Chem. Phys. 21, 6828 (2019)]. We found that the larger ionic radius [rLi+<rNa+<rK+] and higher concentration disturb the hydrogen-bond network of water, resulting in more disordered cationic hydration shell. Our abinitio simulation data for solute concentrations ranging between 1 and 10M can be used to guide future elastic and inelastic neutron-scattering experiments.

The structure of appalachian coal: Experiments and Ab initio modeling

This study focuses on the construction of small-scale atomistic representations of three Appalachian coals of different ranks– high volatile bituminous, low volatile bituminous, and semi-anthracite. Ultimate, proximate, 13C NMR, FTIR, and XPS data were used to infer 2D and construct multiple 3D molecular models for each coal. A new simulation strategy for structure optimization, termed “Sectioned Optimization”, is introduced to ensure energetically stable configurations of the 3D coal models. Density functional methods are applied to the models, improving their quality, and giving insight into the limitations of empirical simulations in this complex system. The structural models were validated ex post facto using a density functional code, VASP. We describe the distribution of local atomic environments in the models to emphasize the significant structural distinctions between the three coal ranks. We also report the first ab initio vibrational density of states calculations for coal and compare it to FTIR and Inelastic Neutron Scattering experiments and identify vibrational modes using ab initio calculations. We also provide animations of the vibrational normal modes in coal that are consistent with FTIR band assignments. We show that the specific heat capacity compares favorably with experimental data.

Crystalline-amorphous-crystalline two-step phase transformation and the resulting supra-nano structure in a metastable iron-based alloy

In this work, we report a crystalline-amorphous-crystalline (CAC) two-step solid-state phase transformation (SSPT) in a 15Cr-2Ni iron-based alloy that results in nanostructures. Microstructurally, the CAC process leads to a localized supra-nano structure having almost equal content of an amorphous matrix and nanocrystalline inclusions less than 6 nm. Dynamically, it is manifested as a glass-transition-like variation of Young's modulus with a sharp surge in internal friction. Through in situ heating and Neutron Scattering experiments, a reduction in the intensities of the austenite diffraction peaks is observed, which is not associated with any intensity increase in other peaks, suggesting the collapse of austenite lattice during heating. The CAC two-step SSPT occurs at temperatures far below the melting point, and the introduction of the dislocations is the indispensable condition giving rise to such a transition. Based on experimental results, we suggest the physical picture of CAC two-step SSPT and a phenomenological model to describe it. Through the analysis, we argue that the CAC two-step SSPT provides a new route to fabricate nanostructured alloys.

Synergism and molecular mismatch in rhamnolipid/CTAC catanionic surfactant mixtures

Rhamnolipids represent a promising class of biosurfactants, whose industrial exploitation could potentially reduce the ecological impact of surfactant-based formulations. In this direction, the rational design of rhamnolipid mixtures with other surfactants constitutes a strategic field of research.In this work, we combine mono-rhamnolipid (mRha) with the cationic surfactant cetyltrimethylammonium chloride (CTAC). The study is conducted at pH = 7.1, so that the mRha carboxylic group is dissociated and catanionic mRha-CTAC mixtures form. The system is investigated across the entire composition range by surface tension and conductivity measurements, which show a strong synergism between the two surfactants in forming mixed aggregates, specifically in mRha-rich mixtures. No coacervation or precipitation is observed, due to the strong asymmetry between the surfactant molecular architectures, with mRha presenting a bulky headgroup and two relatively short tails compared to the single long tail of CTAC. Dynamic Light Scattering (DLS) and Small Angle Neutron Scattering (SANS) experiments show that, while the single surfactants predominantly form small micelles, their mixing results in the spontaneous formation of vesicles. Within the vesicle bilayer, the termini of the longer surfactant tails fold, assuming disordered conformations, as probed by Electron Paramagnetic Resonance (EPR) measurements.These results point to the rational design of rhamnolipid-based catanionic mixtures as a suitable tool to optimize their structural properties, constituting a solid basis for their practical applications.

High-Transmission Neutron Optical Devices Utilizing Micro-Machined Structures

Neutrons are a powerful probe in material science with unique penetrating abilities. A major challenge stems from the fact that neutron optical devices are limited to refractive indices on the order of n≈1±10−5. By exploiting advances in precision manufacturing, we designed and constructed micro-meter period triangular grating with a high-aspect ratio of 14.3. The manufacturing quality is demonstrated with white-light interferometric data and microscope imaging. Neutron-scattering experiment results are presented, showing agreement with refraction modelling. The capabilities of neutron Fresnel prisms and lenses based on this design are contrasted with existing neutron focusing techniques, and the path separation of a prism-based neutron interferometer is estimated.

A SCALABLE TRANSFORMER MODEL FOR REAL-TIME DECISION MAKING IN NEUTRON SCATTERING EXPERIMENTS

The U.S. Department of Energy's (DOE's) neutron research facilities at Oak Ridge National Laboratory (ORNL), including the High Flux Isotope Reactor (HFIR) and the Spallation Neutron Source (SNS), are a state-of-the-art neutron scattering facility that allows researchers to study the structure and dynamics of materials at the atomic scale. At the SNS, neutrons are measured using the time-of-flight (TOF) technique as they move through a neutron beamline to interact with a sample. Large volumes of neutron scattering data are collected and recorded in neutron event mode. Optimal productivity of the TOF instrument is limited due to the lack of real-time data analysis tools. The large amount of data generated by the experiments can be challenging to process and analyze in real time, particularly for experiments that require rapid feedback and adjustment of experimental parameters. The regular computer/workstation cannot keep up with the experiment speed to provide real-time feedback to adjust experimental parameters, so connecting the supercomputers available to the neutron facility is necessary to achieve real-time data analysis and experiment steering. To address this challenge, we exploit the Frontier supercomputer at Oak Ridge Leadership Computing Facility (OLCF) to train a scalable temporal fusion transformer model for real-time decision making of TOF neutron scattering experimentation. In this paper, we present the results using Frontier to provide the processing power needed to rapidly process and analyze large volumes of single-crystal diffraction data collected at TOPAZ, a neutron time-of-flight Laue single-crystal diffractometer at the SNS.

Quasi Elastic Neutron Scattering model library

This paper reports on the development of a collection of dynamical models of one-dimensional peak profile functions used to fit dynamic structure factors S (Q, ħω) of Quasi Elastic Neutron Scattering (QENS) data. The objective of this development is to create a maintainable and interoperable Python library with models reusable in other projects related to the analysis of data from Quasi Elastic Neutron Scattering experiments. The ambition is that the library also will serve as a platform where scientists can make their models available for others. We illustrate how the library can be used by newcomers to the field as well as by experts via different examples. These examples, provided as Jupyter notebooks, show how the QENS models can be integrated in the whole QENS data processing pipeline.

Thermal and mechanical properties of the clathrate-II Na24Si136

Thermal expansion, lattice dynamics, heat capacity, compressibility, and pressure stability of the intermetallic clathrate ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$ have been investigated by a combination of first-principles calculations and experimentation. Direct comparison of the properties of ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$ with those of the low-density elemental modification ${\mathrm{Si}}_{136}$ provide insight into the effects of filling the silicon clathrate framework cages with Na on these properties. Calculations of the phonon dispersion only yield sensible results if the Na atoms in the large cages of the structure are displaced from the cage centers, but the exact nature of off-centering is difficult to elucidate conclusively. Pronounced peaks in the calculated phonon density of states for ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$, absent for ${\mathrm{Si}}_{136}$, reflect the presence of low-energy vibrational modes associated with the guest atoms, in agreement with prior inelastic neutron-scattering experiments and reflected in marked temperature dependence of the guest atom atomic displacement parameters determined by single-crystal x-ray diffraction. The bulk modulus is only weakly influenced by filling the Si framework cages with Na, whereas the phase stability under pressure is significantly enhanced. The room-temperature linear coefficient of thermal expansion (CTE) is nearly a factor of 3 greater for ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$ compared to ${\mathrm{Si}}_{136}$. Negative thermal expansion (NTE), observed in ${\mathrm{Si}}_{136}$ below 100 K, is noticeably absent in ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$. In contrast to ${\mathrm{Si}}_{136}$, the thermal expansion behavior in ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$ is relatively well described by the conventional Gr\"uneisen-Debye model in the temperature range of 10--700 K. First-principles calculations in the quasiharmonic approximation correctly predict an increase in high-temperature CTE with Na loading, although the increase is less than observed in experiment. The calculations also fail to capture the absence of NTE in ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$, perhaps due to anharmonic effects and/or inadequateness of the ordered structural model.

Sleuthing out exotic quantum spin liquidity in the pyrochlore magnet Ce2Zr2O7

The search for quantum spin liquids—topological magnets with fractionalized excitations—has been a central theme in condensed matter and materials physics. Despite numerous theoretical proposals, connecting experiment with detailed theory exhibiting a robust quantum spin liquid has remained a central challenge. Here, focusing on the strongly spin-orbit coupled effective S = 1/2 pyrochlore magnet Ce2Zr2O7, we analyze recent thermodynamic and neutron-scattering experiments, to identify a microscopic effective Hamiltonian through a combination of finite temperature Lanczos, Monte Carlo, and analytical spin dynamics calculations. Its parameter values suggest the existence of an exotic phase, a π-flux U(1) quantum spin liquid. Intriguingly, the octupolar nature of the moments makes them less prone to be affected by magnetic disorder, while also hiding some otherwise characteristic signatures from neutrons, making this spin liquid arguably more stable than its more conventional counterparts.

Anisotropic spin-wave excitations in multiferroic BiFeO3

Polarized inelastic neutron-scattering experiments have been performed to elucidate the anisotropic behavior of the low-energy spin-wave excitations in a multiferroic ${\mathrm{BiFeO}}_{3}$, which shows a cycloidal spin structure below 640 K. Using neutron polarization analysis for single magnetic domain crystals, magnetic excitation modes in and out of the cycloidal plane below 6 meV were separated successfully. The magnetic excitation spectra were analyzed using linear spin-wave theory. The low-energy magnon density of states consist of several magnon modes, including the two anisotropic modes, $\mathrm{\ensuremath{\Phi}}$ and $\mathrm{\ensuremath{\Psi}}$ modes, distributed in and out of the cycloidal plane, respectively, which were previously observed using optical spectroscopies. Furthermore, there are other magnon modes that are not active in optical measurements. A model spin Hamiltonian, which reproduces the spin-wave frequencies observed using optical spectroscopies, explains the overall spectra reasonably well.

Bin-Width Optimization for Multidimensional Histogram and Its Application to Inelastic Neutron Scattering Experiments

Bin-Width Optimization for Multidimensional Histogram and Its Application to Inelastic Neutron Scattering Experiments

Role of the third dimension in searching for Majorana fermions in α−RuCl3 via phonons

Understanding phonons in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ is critical to analyze the controversy around the observation of the half-integer thermal quantum Hall effect. While many studies have focused on the magnetic excitations in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$, its vibrational excitation spectrum has remained relatively unexplored. We investigate the phonon structure of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ via inelastic neutron-scattering experiments and density-functional-theory calculations. Our results show excellent agreement between experiment and first-principles calculations. After validating our theoretical model, we extrapolate the low-energy phonon properties. We find that the phonons in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ that either propagate or vibrate in the out-of-plane direction have significantly reduced velocities and therefore have the potential to dominate the observability of the elusive half-integer plateaus in the thermal Hall conductance. In addition, we use low-energy interlayer phonons to resolve the low-temperature stacking structure of our large crystal of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$, which we find to be consistent with that of the $R\overline{3}$ space group, in agreement with neutron diffraction.

Tuning curvature and phase behavior of monoolein bilayers by epigallocatechin-3-gallate: Structural insight and cytotoxicity

The use of glyceryl monooleate (GMO)-based nanoparticles has not yet been explored in overcoming the low bioavailability of Epigallocatechin-3-gallate (EGCG), a green tea polyphenol with a known anticancer activity. Since the inclusion of a guest molecule can affect the curvature and the supramolecular structure of fully hydrated GMO-based phase, the phase behavior of bulk and dispersed liquid crystalline systems containing EGCG were explored by Small Angle Neutron Scattering and X-Ray Diffraction experiments. Molecular Dynamic Simulations showed how the interaction of EGCG with polar heads of GMO strongly affects the curvature and packing of GMO phase. The EGCG encapsulation efficiency was determined in the nanodispersions and their size studied by Dynamic Light Scattering and Atomic Force Microscopy. A nanodispersed formulation has been optimized with a cytotoxic effect more than additive of GMO and EGCG.

Influence of Magnesium on the Structure of Complex Multicomponent Silicates: Insights from Molecular Simulations and Neutron Scattering Experiments.

A series of multicomponent glasses containing up to five oxides are studied using classical molecular dynamics simulations and neutron scattering experiments. The focus is on the role of magnesium in determining the structural properties of these glasses and the possible mixed effect during a sodium/magnesium substitution. Calculated structure functions (pair correlation function and structure factor) rather accurately reproduce their experimental counterpart, and we show that more fine structural features are qualitatively reproduced well, despite some discrepancies in the preferential spatial distribution between sodium and magnesium to aluminum and boron, as well as the nonbridging oxygen, distribution. The simulated systems offer a solid basis to support previous experimental findings on the composition-structure relationship, allowing for further analysis and property calculation. It is confirmed that the substitution of sodium by magnesium leads to the decrease of four-fold boron and a modification of the alkali coordinations with a significant change of the network structure. Specifically, magnesium coordination extracted from numerical simulations highlights a potential dissociation from penta- to tetra- and hexahedral units with increasing MgO contents along the glass series, which could not be resolved experimentally.

Neural Network Quantum Molecular Dynamics, Intermediate Range Order in GeSe2, and Neutron Scattering Experiments.

A remarkable property of certain covalent glasses and their melts is intermediate range order, manifested as the first sharp diffraction peak (FSDP) in neutron-scattering experiments, as was exhaustively investigated by Price, Saboungi, and collaborators. Atomistic simulations thus far have relied on either quantum molecular dynamics (QMD), with systems too small to resolve FSDP, or classical molecular dynamics, without quantum-mechanical accuracy. We investigate prototypical FSDP in GeSe2 glass and melt using neural-network quantum molecular dynamics (NNQMD) based on machine learning, which allows large simulation sizes with validated quantum mechanical accuracy to make quantitative comparisons with neutron data. The system-size dependence of the FSDP height is determined by comparing QMD and NNQMD simulations with experimental data. Partial pair distribution functions, bond-angle distributions, partial and neutron structure factors, and ring-size distributions are presented. Calculated FSDP heights agree quantitatively with neutron scattering data for GeSe2 glass at 10 K and melt at 1100 K.

Soft Matter Sample Environments for Time-Resolved Small Angle Neutron Scattering Experiments: A Review

With the promise of new, more powerful neutron sources in the future, the possibilities for time-resolved neutron scattering experiments will improve and are bound to gain in interest. While there is already a large body of work on the accurate control of temperature, pressure, and magnetic fields for static experiments, this field is less well developed for time-resolved experiments on soft condensed matter and biomaterials. We present here an overview of different sample environments and technique combinations that have been developed so far and which might inspire further developments so that one can take full advantage of both the existing facilities as well as the possibilities that future high intensity neutron sources will offer.

Analysis and Comparison of Quasi-Elastic Neutron Scattering (QENS) Spectra Model

The Quasi-Elastic Neutron Scattering (QENS) experiment is a novel method to study confined water dynamics by measuring the neutron energy transfer ( E )and scattering vector (Q) after the neutron interacts with the sample target. By adopting a suitable theoretical model and performing nonlinear least-squares fitting analysis on the Quasi- Elastic Neutron scattering spectra, the relevant physical parameters describing the water dynamics can be obtained, and then the constrained water dynamics in the sample can be detected. There are many types of Quasi-Elastic Neutron Scattering spectra fitting analysis models, and the fitting effects of different models and the relevant physical parameters obtained to characterize water dynamic information are not the same. This paper uses the Jump-diffusion and Rotating-diffusion Model (JRM) and the revised Jump-diffusion and Rotating-diffusion Model (rJRM) to fit the QENS spectra of pure magnesium silicate (M-S-H) and pure calcium silicate (C-S-H) samples, respectively. The measurement temperatures of QENS spectra from M-S-H and C-S-H samples are 210K up to 280K and 230K up to 280K, respectively, the E ranges from -120 to 120μev and the Q value 0.3 A^(-1)~1.9 A^(-1). Different physical parameters describing the dynamic information of confined water can be obtained by using the two models for QENS spectra fitting analysis. Compared with the JRM model, the physical parameters obtained by using the rJRM model are more comprehensive, and it is suitable for all neutron energy transfer and scattering vector measurements. From the qualitative point of view, the QENS spectra fitting curve of rJRM model is more consistent with the original data points. From the quantitative point of view, the chi-square value fitted by the rJRM model is more consistent with the mathematical statistics law than the JRM model, and the time required for nonlinear fitting is shorter.

Impact of the neutron-depolarization effect on polarized neutron scattering in ferromagnets.

It has been known for decades that a ferromagnetic sample can depolarize a transmitted neutron beam. This effect was used and developed into the neutron-depolarization technique to investigate the magnetic structure of ferromagnetic materials. Since the polarization evolves continuously as the neutrons move through the sample, the initial spin states on scattering will be different at different depths within the sample. This leads to a contamination of the measured spin-dependent neutron-scattering intensities by the other spin-dependent cross sections. The effect has rarely been considered in polarized neutron-scattering experiments even though it has a crucial impact on the observable signal. A model is proposed to describe the depolarization of a neutron beam traversing a ferromagnetic sample, provide the procedure for data correction and give guidelines to choose the optimum sample thickness. It is experimentally verified for a small-angle neutron-scattering geometry with samples of the nanocristalline soft-magnet Vitroperm (Fe73Si16B7Nb3Cu1). The model is general enough to be adapted to other types of neutron-diffraction experiments and sample geometries.

Multi-Scale Simulations and Neutron Scattering Experiments Reveal Dynamical Properties of the Bacterial Cytoplasm Near Cell-Death Temperature

Life on Earth exhibits an amazing adaptive capacity to a vast range of temperatures. While the molecular mechanisms underlying such adaptability are not yet fully understood, it has been proposed that the cell-death temperature coincides with a denaturation catastrophe of the proteome. Here we use multi-scale simulations and neutron scattering experiments to describe the dynamical properties of proteins inside a bacterial cytoplasm when approaching thermal denaturation. Through a combination of extensive coarse-grain and all-atomistic simulations, we characterize distinct contributions to the picosecond- and nanosecond dynamics of proteins immersed in a crowded and heterogeneous environment mimicking the composition of E.

Hybridized quadrupolar excitations in the spin-anisotropic frustrated magnet FeI2

Magnetic order is usually associated with well-defined magnon excitations. Exotic magnetic fluctuations with fractional, topological or multipolar character, have been proposed for radically different forms of magnetic matter such as spin-liquids. As a result, considerable efforts have searched for, and uncovered, low-spin materials with suppressed dipolar order at low temperatures. Here, we report neutron-scattering experiments and quantitative theoretical modeling of an exceptional spin-1 system -- the uniaxial triangular magnet FeI2 -- where a bright and dispersive band of mixed dipolar-quadrupolar fluctuations emerges just above a dipolar ordered ground-state. This excitation arises from anisotropic exchange interactions that hybridize overlapping modes carrying fundamentally different quantum numbers. Remarkably, a generalization of spin-wave theory to local SU(3) degrees of freedom accounts for all details of the low-energy dynamical response of FeI2 without going beyond quadratic order. Our work highlights that quantum excitations without classical counterparts can be realized even in presence of fully developed magnetic order.